Partial response
The tumours have shrunk substantially (by at least 30% in total diameter) but not disappeared. Together with complete responses it makes up a trial's response rate.
RECIST 1.1 defines partial response as at least a 30% decrease in the sum of the longest diameters of the target lesions with no new lesions; the response must be confirmed on a later scan. Response rate (complete plus partial) is the quickest signal that a drug is active and the basis of many accelerated approvals, but shrinkage does not always translate into longer life, and some effective drugs, notably immunotherapies and some targeted agents, produce long stable disease rather than shrinkage. How long a response lasts (duration of response) is often more informative than how many patients achieve one.